Icarus: The Motivations for Fusion

by Paul Gilster on January 21, 2010

If you haven’t read George Dyson’s fascinating history of Project Orion, let me recommend it to you highly. Project Orion: The True Story of the Atomic Spaceship (Henry Holt, 2002) fires the imagination with the audacity of the project, a nuclear pulse rocket that would have exploded atomic bombs behind the vehicle, using the world’s ultimate shock absorbers to ride the wave to the outer planets. There was talk of going to Saturn (to Enceladus, no less) in the late 1960’s, but those dreams were quickly quashed by treaties forbidding nuclear testing.

The Problem with Orion

Kelvin Long, who heads up the ambitious Project Icarus attempt to revisit and extend Project Daedalus, notes in a recent post on the Icarus blog that Freeman Dyson (George’s father) ultimately gave up on Orion (a fact that surprised me when I did a telephone interview with him on the prospects for interstellar propulsion back in 2003). Here’s what Dyson says about the subject in his book Disturbing the Universe (1979):

Sometimes I am asked by friends who shared the joys and sorrows of Orion whether I would revise the project if by some miracle the necessary funds were suddenly to become available. My answer is an emphatic no…. By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…. Many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.

People still advocate concepts like Orion, but the Icarus team draws on the extensive work that has been done on pulsed fusion using a high intensity laser beam or an electron driver to produce detonation of fusion fuel pellets. This was the Daedalus approach in the 1970s, capitalizing not only on the research performed for Orion but also the intense scrutiny on pulsed micro-explosions that began in the early 1970s. The crucial fact is that this body of knowledge can be tapped to produce realistic performance assessments.

Image: Daedalus arrives at Barnard’s Star. Credit: Adrian Mann.

Fusion Alternatives and Their Uses

Long goes through various space propulsion systems in his post, noting two obvious categories. The first involves electric propulsion, which is power-limited; i.e., these methods can produce high specific impulse and high exhaust velocity, but at very low levels of thrust. The second covers both chemical and nuclear methods, which are energy-limited and can produce high thrust with high exhaust velocity, although at the cost of a short specific impulse. If we can develop fusion technologies sufficiently, we should be able to combine the advantages of both these approaches, getting more out of our fuel and increasing efficiency.

The fuel itself? Long talks about the same Deuterium/Helium-3 reaction that interested the Daedalus designers, although he notes the potential for Deuterium/Tritium. The latter involves the production of large amounts of neutrons and could therefore be problematic. On the other hand, we’re talking about huge demands on the civilization that develops D/He3 methods, too, as he goes on to note when discussing recent advances in fusion research:

The physics of fusion research has moved forward dramatically in recent years with the US National Ignition Facility now operational and others such as Laser MegaJoule in France under construction. Fast ignition proposals such as HiPER are also under consideration. The chances for scientists finally solving the ‘fusion problem’ are greatly increased. With this in mind, thinking about the implications to deep space missions is timely. It is quite possible that the demands of a fusion based drive will necessitate a sophisticated space based infrastructure for resource acquisition, processing, manufacture and construction. Especially if He3 mining of the gas giant Jupiter or even the Moon is considered. However, as a theoretical exercise in the application of science and engineering Project Icarus has a large amount of intrinsic worth.

Icarus and Competing Studies

We’ll see what the Project Icarus team comes up with (disclaimer: I am a consultant on the project, and obviously an advocate of Icarus). Given that Daedalus was a design study that changed the game, producing the first detailed investigation of what it would take to build an interstellar probe, it makes sense to build upon that work to re-evaluate its key assumptions and improve the design. It would be excellent to see such efforts paralleled by a study of an interstellar probe based on solar sail technology coupled with laser or microwave beam propulsion. Icarus is one attempt to expand our knowledge of interstellar engineering. It is not meant to rule out studies of competing concepts by others.

Ultimately [writes Long] the aim would be to improve the Technological Readiness Level for this sort of engine design type. If other teams used the same approach, and say built upon historical projects like Vista, Longshot, TAU or Starwisp it is a personal belief that the credibility of engineering designs for interstellar missions would be vastly improved. The historical link with both Orion and Daedalus also captured the hearts of the Icarus team and made for a strong support base upon which to galvanize both academic and public interest; a necessary condition to inspire people that this design study is worth doing. Although it is also true that after having questioned the original assumptions of Daedalus, the final Icarus design may look very different with technology not envisioned in the 1970s.

Indeed. Are hybrid designs possible? Long mentions antimatter-catalyzed fusion techniques (Penn State did early work on these possibilities), and that notion resonates given the relatively small amounts of antimatter needed and the potential for mining naturally occurring antimatter in the Solar System (James Bickford has studied the matter extensively — see this earlier Centauri Dreamsstory for more on his work). Whatever the case, just as the solar sail concept was refined through key papers in the 1980s, leading to our current readiness to deploy small sails for testing in space, fusion methods continue to be investigated in hopes of pushing our research into designs that will eventually be practical.

I’m not convinced that the radiation hazard from Orion is a serious issue for interstellar starships. For the original Orion design that would have used nuclear pulse propulsion even for launch from Earth’s surface perhaps this would be a serious concern, but such designs are unnecessary and wasteful. Any reasonable serious interstellar Orion design will save the use of “nuclear pulse units” (thermonuclear weapons) for the main interstellar journey, using more mundane propulsion for assembling the craft in interplanetary space. Given this, the exhaust from Orion would be directed into interplanetary space, likely far removed from Earth and other habited planets, and would likely have escape velocity from our solar system. Even if the exhaust happened to stick around for a while it’s highly questionable that it would increase the radiation hazard of the inner solar system.

The worst case scenario would be that a substantial amount of the exhaust fallout would end up in a biosphere somewhere, but that seems more than easy enough to prevent with simple logistics.

Interesting post, I happened to be reading today Prospects for Interstellar Travel by John Mauldin 1992 (got a 2nd hand copy yesterday in the mail from Amazon) and really getting stuck into it, I’ll write up a book review when I finish reading it. The 2nd chapter that I was upto today mentions about the Daedalus and Orion projects and mentions that only 1/10 of the bomb energy contributes to forward thrust accelerating the ship to only 0.03c.

Not sure if this was posted here before but there’s an interesting essay on the not so viable warp drive concept also worth a read “On the impossibility of superluminal travel: the warp drive lesson “:http://www.fqxi.org/community/forum/topic/539

It appears that the case for FTL travel is a nogo with current physics however until we have a working theory explaining the effects of matter/energy on the vacuum and viceversa (gravity for eg), the case is by no means closed.

By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…. Many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.

WT…. ? As if universe outside of the protective shield of a thick atmosphere were something else than one big radioactive mess. Radiation phobia at takes its toll even on smartest scientists.

Paul, the Mauldin book is essential. You’ll find that it’s extremely useful for its bibliography and notes, which I used a lot while writing the Centauri Dreams book, and although it’s now dated, it should be on the shelf of anyone with a serious interest in interstellar flight. Would be glad to have your review of it when you’re done.

I worked in fusion at Livermore 1967-71, and quit because I thought tokomak and laser fusion (which Teller urged me to join) would fail. So far they have. Laser fusion in 1970 was touted as “5 years away” and acknowledged as a bomb simulation program–which it still is. We’ve spent tens of billions on it, a scandal–and Livermore still says it’s a possible power source, though it would require a shot/sec to produce 100 kW, if it ever worked.
Nuclear reactor rockets make more sense now. Fusion of proton+Boron is the real goal (producing only alpha particles), not the neutron source of He3 & tritium, etc
This wasteful history is a tragedy, not a triumph.

“WT…. ? As if universe outside of the protective shield of a thick atmosphere were something else than one big radioactive mess. Radiation phobia at takes its toll even on smartest scientists.”

The politics and public perception of airborne nuclear materials are not to be trifled with, irrespective of the actual facts. Remember the brouhaha over Cassini’s RTG, and the overblown fears regarding the “fallout” of a possible launch failure, or even gravity-assisted maneuvers around Earth of spacecraft carrying (or not!) RTGs.

The ignition temperatures for proton+Boron are too high. Except in a weapon, they can barely get tritium+deuterium to fuse. My guess is that deuterium+deuterium is our only real technological option for a propulsion scheme.

The “Mike” device was based upon a deuterium+deuterium reaction and conventional thermonuclear explosives are based upon tritium+deuterium with tritium created from lithium. Has any other fusion reaction been used in an explosive device, e.g. proton+boron?

Fusion of proton+Boron is the real goal (producing only alpha particles),

Can the fusion be ignited by simply impacting protons on a solid boron piece ? If it can, then it might have some use as a fusion fragment engine ( shoot protons into a thin disc and let the resulting alpha particles propel the ship ). If not, then it is, I am afraid, a waste of time, because the plasma confinement requirements are orders of magnitude higher than for other nuclear reactions, straight in the “never-neverland” I am afraid.

I attended a particle beam conference in 1997 and met a lot people who quit the fusion program for the same reason as Gregory Benford. It has been obvious for a long time that both the Tokamak and the laser fusion are fraudulent programs. They will never lead to commercial fusion power. I think the laser fusion program is really a cover for continued nuclear weapons R&D. Many of the people I met at this conference were quite disgusted with both of these programs and thought they should be shut down. They felt resentment at having spent years of their lives on what they had come to view as a giant fool’s errand. Many of them felt that these projects were nothing more than “pork”.

Bussard’s IEC polywell (EMC2) and Rostakor’s CB/FRC (Tri-Alpha) are the two B11-H fusion concepts that I am aware of. There may be others. The problem with B11-H fusion is that it cannot be done in a Maxwellian (thermal) plasma even if you can achieve the confinement conditions because of the Bremstrahlung losses. Its not clear to me if either the IEC polywell or Tri-Alpha’s FRC (both of which use non-Maxwellian plasma) concepts will work.

Perhaps one option is the “fusion runway” that uses high relative velocity between a vehicle and a fusion pellet to induce the tougher kinds of fusion. Imagine boron microsails being zapped by proton beams so the fusion products push against the vehicle’s mag-field.

By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…. Many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.

WT…. ? As if universe outside of the protective shield of a thick atmosphere were something else than one big radioactive mess. Radiation phobia at takes its toll even on smartest scientists.

Agreed. I sense a lazy old bastard conforming to non-scientific political propaganda. We don’t even need nuclear fusion- we have 65 years of fission engineering. Why wait another 60 years for fusion to go from the experimental to the engineering stage? Now with thorium and 4th generation reactors we could have a design that would outperform the 50’s and 60’s concepts.

It’s the long term fuel supply. A solar economy would run out of uranium quickly; thorium more slowly, but still likely to choke off our prospects in millenia.

If we can generate and store antimatter safely, well and good; but it still ties us to the Sun.

If we could burn lithium or boron in fusion reactors, we could mine the outer planet moons and KBOs for a long time. But the real prize would be CNO-II cycle fusion, burning protons to helium; perhaps in an advanced polywell or similar.

As I understand the Bremsstrahlung problem, it applies very generally for “small” systems, i.e. those much smaller than a star. As long as, as Kurt mentions, the plasma is thermal. That means NO amount of pressure or heat can EVER get the “tougher kinds” to break even. Non-Maxwellian methods seem to offer a way out, but none are known that do not have different, equally unsurmountable obstacles in their way. Polywell is going to die a fairly quick death with Bussard gone (albeit it may be made a comfortable one with DARPA funds). Colliding beam approaches are dealt with convincingly here: http://www.sciencemag.org/cgi/content/full/281/5375/307a.

My impression is that non-thermal methods generally don’t work as a matter of principle, because the fusion cross-section is much smaller than the Coulomb scattering cross-section, such that ions are either lost or thermalized well before they fuse, regardless of which method is used to accelerate and contain them.

Muon catalyzed fusion is similarly tantalizing, but a few orders of magnitude out of reach, because muons decay before they can break even in catalyzed fusion events.

Somebody here said that both the tokamak and laser programes dedicated to fusion energy were fraudulent programmes. Have patience !
The ITER programme npow building at CVadarachje France is certainly very expensive and also faces huge technical challenges, but with time is loikely to succeed. Laser fusiopn, although the “new kid on the block” is now advancing so fast that it will soon be leaving tokamak behind. The National Ignition Facility in Lawrence Livermore California exists for two purposes… maintenance of USA’s nuclear stockpile (hydrogen bombs don’t last forever, just sitting on the shelf) and (much more important today) achieving controlled fusion with an energy break-even. Within 3 years the NIF is probably going to have proved that fusion can be triggered with large lasers, then the European HiPER Project steps up to develop the knowledge into something from which a working prototype fusion power plant can be built.
This is not quick work and it certainly isn’t cheap but, compared with the cost of not doing it, fusion energy research is a real bargain. We all know how soon the fossil fuels are going to start running out and how much of a mess we are making of our environment. We have seen how governments which rejected nuclear fission have already realised that buuilding another generation of fission reactors is the only way to prevent a huge energy crisis. To prattle on about fraud at this stage is almost suicidal. If we don’t crack fusion energy within the next 30 years mankind is heading for absolute disaster… straight from the boom centuries fuelled by cheap energy from coal, oil and gas… back to darker times. What a pity it would be to have come so close to solving earth’s energy problem for the truly long term, only to havbe the possibility snatched away from us by a few howling “fraud” ! Mankind and the earth itself need us to crack fusion energy and really soon. We must continue to do it… for our children and grandchildren !

With really fine-scale guidance one could aim a ‘snowflake’ of hydrogen at the incoming boron sail. Might improve the reaction rate enough to make the system useful. Main impediment is the need for a micro-sail accelerator. for example, Jordin Kare’s Sail-Beam relies on extremely high transmissivity of the material so most of the energy passes harmlessly through the sail, a fraction is reflected back and a miniscule amount absorbed.

It appears that the case for FTL travel is a nogo with current physics however until we have a working theory explaining the effects of matter/energy on the vacuum and viceversa (gravity for eg), the case is by no means closed.

Insufficient work has been done on Mach-Einstein theories.

Polywell is going to die a fairly quick death with Bussard gone

Rick Nebel is working on it. Funded by the US Navy.

So far: WB-7 (a replication of WB-6 with small improvements)

WB-8 (now in operation)

WB-9 (on the drawing board)

As to death of the project? It depends on results. The nice thing is that we should have an answer by about May of 2011. Go or no go.

Plasma Physicist Dr. Nicholas Krall said, “We spent $15 billion dollars studying tokamaks and what we learned about them is that they are no damn good.”

There is one nation which has a sophisticated space program, lots of
nuclear bombs, lots of people and other resources, one of the few
thriving economies on this planet, and vast tracts of harsh, uninhabited
territory that would be ideal for detonating nuclear bombs with minimal
danger to human and other life (and they have).

This nation, which has existed in various forms for thousands of years
and for a long while was technologically superior to any place else, has
also ignored the Nuclear Test Ban Treaty of 1963 and will probably
continue to do so for the foreseeable future. Not only have they talked
about establishing manned bases on the Moon and Mars (and will have
their first Earth-orbiting space station in LEO by this year or next), but
they have even made mention of interstellar missions in 2009.

Sure, it may be rhetoric for now, but they do possess certain capabilities in
these areas. What I am saying is they may be the ideal nation to launch an
Orion vessel and they won’t be fretting over the concerns others do, some
of which is justified while others seem to be green because it is the PC trendy
thing to do right now.

So while as much as I would like to see controlled fusion work both for
interstellar vessels and to power our civilization, I am ambivalent about
seeing another study that will declare fusion ready in yet another twenty
years. We do have the means to get a vessel to the nearest star systems
within a century or so right now and if certain groups don’t pick up on
this, I can think of at least one nation that could. Whether you think this
is a good idea or not depends on your politics, which once again cannot
be ignored in such an undertaking as interstellar travel.

I can be fairly certain in the belief that had the Cold War not pushed the
US and USSR to get humans on the Moon by 1969, there would be
academic groups still talking to this very day about how to go about
landing astronauts on the lunar surface, just as we are still talking
about putting humans on Mars when we could have done this back in
the 1980s if we really wanted to.

Note that environmental groups which once went ballistic at the mere
mention of the word nuclear are now quietly working it into their agenda
for a clean, green world, since it does have numerous advantages over
fossil fuels, which are both messy and finite.

I lost all respect for anti-nuke groups during their tireade agains the
launch of the Cassini probe to Saturn back in 1997. One fanatic in
particular showed a profound lack of scientific understanding. When
Cassini was scheduled to perform a flyby of Venus in 1999 for a
gravity boost, he suggested the probe should be smashed into that
planet because Venus had no atmosphere. When I corrected him on
this slight error, plus added that some scientists think Venus might
have some life forms on it despite the seemingly harsh environment,
he essentially replied with better Venus than Earth.

But what really sent me over was the report from a friend who attended an
anti-Cassini rally in Cambridge, Mass. just before the RTG laden probe
was sent aloft (without incident) in 1997. As the only scientist there on
the pro-Cassini side, he was told by the hostile crowd that they had already
made up their minds about the Saturn explorer and were not going to be
swayed by facts.

So, while I know nuclear bombs are not nice things and that radiation can
be a hazard if handled incorrectly, launching Orion from a very remote
region on Earth that will never be developed or better yet in radiation
soaked space still looks like our best bet if we ever want to get to the stars
realistically any time soon. Otherwise we will still be fifty years later talking
about what might happen one hundred years from then.

James Makepeace said:
“The ITER programme npow building at CVadarachje France is certainly very expensive and also faces huge technical challenges, but with time is loikely to succeed.”

I am also skeptical about Tokamaks. ITER is too big and too expensive. I don’t think it’s possible for an economical energy production technology to come from ITER. The R&D money spent on ITER should be spent on more promising technologies.

James Makepeace also said:
” Laser fusiopn, although the “new kid on the block” is now advancing so fast that it will soon be leaving tokamak behind. The National Ignition Facility in Lawrence Livermore California exists for two purposes…”

I’ve toured the NIF facility at Lawrence Livermore National Lab and was very impressed. NIF does exist mainly for maintenance of the nuclear weapons program but I believe NIF can also help in better understand inertial confinement nuclear fusion.

James Makepeace said:
“We all know how soon the fossil fuels are going to start running out and how much of a mess we are making of our environment. We have seen how governments which rejected nuclear fission have already realised that buuilding another generation of fission reactors is the only way to prevent a huge energy crisis. To prattle on about fraud at this stage is almost suicidal. If we don’t crack fusion energy within the next 30 years mankind is heading for absolute disaster… straight from the boom centuries fuelled by cheap energy from coal, oil and gas… back to darker times.”

Basing an economy on burning fossil fuels is a “monkey trap” and complete idiocy. If we don’t shift over to nuclear energy soon, the world’s economy could implode and we find ourselves without the economic/industrial resources to transition over to nuclear energy, i.e. we could be forever trapped in an 18th century economy. However nuclear fusion might be a “practical joke from God”, i.e. nuclear fusion is plainly visible as a benevolent proton-proton fusion reaction on the Sun and in its malevolent form as thermonuclear weapons but completely unattainable as an economic controlled energy source. The (assumed) failure of controlled nuclear fusion could be used as an argument about the anthropic principle, i.e. the Laws of Physics do not allow a species to survive beyond its planet of origin. Being an optimist, I don’t buy into this but it is an interesting line of speculation.

If we don’t crack fusion energy within the next 30 years mankind is heading for absolute disaster… straight from the boom centuries fuelled by cheap energy from coal, oil and gas… back to darker times.

three thousand is written with 3 zeros not one. We have enough uranium and thorium for thousands of years if we reprocess fuel, and for hundreds of thousands to millions if we filter it out of the sea water.
Besides that, molten salt reactors can be built by very low tech means because you don’t need any pressure vessel and they have so high negative void coeficient that they are essentially self-regulating without any outside interference. All you need is one big steel vessel to hold the thing together and one pump to pump the salt. The rest is just a big steam engine.
Nothing a 18 century craftsman can’t do if he knows early 20 century physics.

Shooting protons at a boron target would produce fusion, but only very little. The cross section is too small. For every fusion event, thousands (or millions?) of protons will just pass through or be thermalized, depending on the thickness of the target. This costs way more energy than the the single fusion event yields. That’s why the non-Maxwellian methods strive to have each ion go around thousands of times, to get multiple chances. From all I’ve heard, though, the gap is just way too large to close. “Practical joke from god”, indeed, just as with muon catalyzed fusion. While magnetic confinement and laser ignition are not easy, they are what hope we have, and, at least in my opinion, there is enough progress to keep going. Calling it fraud is certainly out of line, especially when coming from those who live off research grants or investor (sucker) money obtained by writing grandiose proposals for fundamentally flawed ideas.

Otherwise I am with T_U_T. Fusion would certainly be nice, but a matter of survival it is not. Despite the wool over everyones eyes (probably put there by the oil industry) there is plenty of energy all around us, waiting to be exploited once we finally get sick (or run out) of the gooey black stuff. Fission is just the most reliable of them, and, if done right, probably the next cheapest after coal. It should be what we use to solve the so-called “energy problem” once and for all. Remember, we were 20% of the way there 50 years ago, the French are at 80%. It is a matter of will, not way.

For every fusion event, thousands (or millions?) of protons will just pass through or be thermalized, depending on the thickness of the target. This costs way more energy than the the single fusion event yields.

So, just shooting protons into a solid boron block does not do it. What about a thin boron sheet, so that the protons don’t get thermalized. Most of them will just pass through or scatter and can be gathered on the other side, accelerated, and used again.

How much energy would be lost that way through scattering the protons off chared particles ? More than the fusion energy ?

Adam said:
“Sweeping aside all the pessimism another approach to fusion is to use that big old Fusion Reactor in the sky and build SPS. Might need to improve our spacelift technology but that’s hardly onerous.”

Achieving Cheap Access to Space (CAtS) has been extremely onerous. There are guys in the aerospace profession who now think it’s impossible. I’m not so pessimistic. I still think a well designed Two-State-to-Orbit (TSTO) system can probably knock our access to LEO down to $1000/kg. However the magic number for dreamy concepts like SPS is $100/kg. For SPS to work it would have to compete economically against nuclear fission, wind power, ground based solar and bio-fuels. Nuclear fission is almost a sure bet economically after the irrational political barriers have been pushed aside.

Eniac said:
“Despite the wool over everyones eyes (probably put there by the oil industry) there is plenty of energy all around us, waiting to be exploited once we finally get sick (or run out) of the gooey black stuff. Fission is just the most reliable of them, and, if done right, probably the next cheapest after coal. It should be what we use to solve the so-called “energy problem” once and for all. Remember, we were 20% of the way there 50 years ago, the French are at 80%. It is a matter of will, not way.”

I agree. However my worry is that we’ll first “fall off the cliff” economically before making the decision to abandon fossil fuels and transition over to nuclear energy.

Concerning protons impacting lithium: A starship needs to have an erosion shield in order to deal with impacting interstellar grains and heating due to free molecular gas. If the erosion shield was composed of boron then it could produce some energy due to nuclear fusion assuming the impacting free molecular gas had enough kinetic energy. This may actually be a bad idea because the erosion shield will get pretty hot simply from impacting free molecular gas. Heat rejection maybe a bigger issue than energy production.

Regarding nuclear powered space craft, Imagine a hydrogen bomb powered photon rocket wherein a pure hydrogen bomb is used to instantly heat a black body solid hollow sphere made of carbon fiber, graphene, or other extremely refractory materials which have a high emissivity.

Now, the specific heat of graphite fiber is 0.71 (kJ/kg K) and so in order to heat one kilogram of graphite up to 3000 K, 1000 kCal of heat is required. A one megaton bomb has a yield of 1,000,000,000 MCal and thus such a device can heat one million metric tons of carbon graphite to a temperature of 3,000 K in an instant. Carbon fiber also has a density equal to 1790 kg/(m EXP 3) and so a sphere made of carbon that has a shell thickness of 1/1.790 meters = 0.55865 meters will have a surface area of one square kilometer and will have a radius of about 282.094 meters.

Now further contemplate that our space craft can detonate another bomb in its chamber after the sphere has cooled to 2,525 K to radiate at [2.2965 x (10 EXP 12)] watts at which point a 0.5 megaton bomb is detonated within the center of the sphere to bring the sphere back up to 3,000 K and the process repeats is self over and over again. Assuming that an average radiative power can be maintained at 3.5 x 10 EXP 12 Watts, which is reflected by a highly directional mirror in one direction such as in a photon rocket cone, the ship will obtain a thrust of [9.2 x (10 EXP – 6)] [3.5 x (10 EXP 12)]/(1,370) Newtons = 23,503.6 Newtons. Now F = Ma and so a = F/M. If we assume a total ship mass of 3 million metric tons including the sphere, the reflector, and main body of the ship, and the use of pure fusion atomic bombs, we obtain an acceleration of (23,503.6)/ [3 x (10 EXP 9)] meters per second squared = 7.8345 x 10 EXP – 6 meters per second squared. In 1,000 years, the space craft will be traveling at 235,000 meters per second. In 50,000 years, the craft could be traveling at a velocity equal to 0.03917 C. In 200,000 years the space craft would reach 0.157 C. These calculations neglect the effects of special relativity since such effects would involve only a small correction at these low end relativistic velocities.

Now a BB radiator that emits 4 x 10 EXP 12 Watts of power radiates at a rate equivalent to the detonation of a one kiloton nuclear device per second or a 0.001 megaton device per second and so it would take a BB radiator radiating at 4 x 10 EXP 12 Watts, 1,000 seconds to radiate the energy produced by a one megaton bomb explosion which is approximately equal to 20 minutes. The detonation of a one megaton device in the center of the sphere would likely vaporize the inner portion of the sphere material and either way, greatly lower the mechanical strength of the sphere causing it to immediately rupture. Thus, we might want to use a one kiloton bomb detonation per second in order to heat and maintain the temperature of the sphere at around 3,000 K.

The use of 1 kiloton nuclear devices seems much more reasonable in order to avoid excessive hydrodynamic shock to the sphere from its inside.

Note that a one megaton pure fusion bomb would have a mass of about 6 kilograms and a one kiloton pure fusion bomb would have a mass of about 6 grams! – a tiny mass indeed and so the development of pure fusion bomb technology needs to be precisely regulated. The famous neutron bomb which is capable of immediately irradiating a small city to death only had a yield of 1 kiloton, thus the reason for extreme caution with mini-nukes.

Now further contemplate that our space craft can detonate another bomb in its chamber after the sphere has cooled to 2,525 K to radiate at [2.2965 x (10 EXP 12)] watts at which point a 0.5 megaton bomb is detonated within the center of the sphere to bring the sphere back up to 3,000 K and the process repeats is self over and over again. Assuming that an average radiative power can be maintained at 3.5 x 10 EXP 12 Watts, which is reflected by a highly directional mirror in one direction such as in a photon rocket cone, the ship will obtain a thrust of [9.2 x (10 EXP – 6)] [3.5 x (10 EXP 12)]/(1,370) Newtons = 23,503.6 Newtons. Now F = Ma and so a = F/M. If we assume a total ship mass of 3 million metric tons including the sphere, the reflector, and main body of the ship, and the use of pure fusion atomic bombs, we obtain an acceleration of (23,503.6)/ [3 x (10 EXP 9)] meters per second squared = 7.8345 x 10 EXP – 6 meters per second squared. In 1,000 years, the space craft will be traveling at 235,000 meters per second. In 50,000 years, the craft could be traveling at a velocity equal to 0.03917 C. In 200,000 years the space craft would reach 0.157 C. These calculations neglect the effects of special relativity since such effects would involve only a small correction at these low end relativistic velocities.

Now a BB radiator that emits 4 x 10 EXP 12 Watts of power radiates at a rate equivalent to the detonation of a one kiloton nuclear device per second or a 0.001 megaton device per second and so it would take a BB radiator radiating at 4 x 10 EXP 12 Watts, 1,000 seconds to radiate the energy produced by a one megaton bomb explosion which is approximately equal to 20 minutes. The detonation of a one megaton device in the center of the sphere would likely vaporize the inner portion of the sphere material and either way, greatly lower the mechanical strength of the sphere causing it to immediately rupture. Thus, we might want to use a one kiloton bomb detonation per second in order to heat and maintain the temperature of the sphere at around 3,000 K.

However, finding some exotic molecular or perhaps some type of crystalline neutron dense nuclear matter type of materials that could reliably radiate at 30,000 K would permit 10,000 times the acceleration thus affording reasonable near star transit time.

Matter antimatter bomb energized 30,000 K emitters would have the same acceleration but would be limited in gamma factor only by the quantity of matter antimatter fuel carried on board and/or harvested from matter antimatter pellet fuel streams, obviously the caveats being the need to neutralize astrodynamic drag, adequately shield the space craft, and the like. Higher radiator temperatures would produce even higher accelerations. The effective Isp for this photon rocket would be 1C expressed in units of C where both matter and antimatter fuel components are carried along from the start of the mission.

T_U_T: If you recapture protons that missed a thin target (99.99… % of them), and accelerate them all over again, you will certainly require more energy than you can get out. They only remotely viable option is to bend them around (with magnetic fields == colliding beam fusion, electric fields == IEC fusion) without loss of energy, and do so many thousands or millions of times.

However, those that miss only barely get their trajectories all twisted out of shape by the Coulomb interaction (scattered, in technical terms…), and will get lost from the recirculating aperture. My guess is that those are still a large enough fraction to make the concept a complete no-go, but I haven’t really researched it at all.

I wonder if the best option for a fusion drive might not be the deuterium-tritium reactions with the neutrons captured in lithium to breed more tritium. That way, we can use the easiest reaction, and do something useful with the neutrons. Lithium is relatively easy to obtain, and, more importantly, easier to store since it does not require a tank structure. Essentially, you’d store the deuterium in tanks made from lithium. That would make for a dry weight of the booster of almost zero.

This would be easiest with a VASIMR style magnetically confined plasma, if that is possible, but it could also work for the mini-bomb scenario.

Ok, I know I’m rambling, but here is a scenario: Imagine a long train of hollow lithium spheres, filled with liquid or frozen deuterium. They’d be all fuel, the only structural mass they’d need is the tether tying them together. Even that might be made of lithium. In front would be the “locomotive”, one or more engines, fusion drives of some sort that burn the deuterium and tritium bred from the lithium and would pull the whole assembly along. We can make this train very, very long, to get otherwise impossible dry/fueled mass ratios and carve out a little more relativistic speed from the rocket equation than Daedalus. Or, alternatively, allow for deceleration at the target.

More rambling: A D-T reaction produces one neutron, and breeding one Tritium consumes one neutron. I suppose the only reason the cycle can be closed is because of D-D side reactions which produce extra tritium. If indeed the cycle can be closed, this means that very few neutrons will actually go to waste, as most are recycled into the tritium. There is no way to avoid thermalizing the neutronic energy, but thermal energy can be recovered up to the Carnot limit, and utilized to accelerate reaction mass (leftover lithum, non-lithium structural materials). Even the waste heat can produce a little thrust if radiated backwards.

All this might serve to make the point that the ideal of aneutronic fusion should not be pursued at the exclusion of much more practical, albeit somewhat less efficient fusion reactions. Imagine you would not have to propose to mine the atmosphere of Jupiter, this alone would make the concept so much more credible.

One more note: The tritium breeding reaction also produces thermalized helium-4, which would probably make up the bulk of the reaction mass for the ion drive (the one that’s needed to utilize the recoverable thermal energy).

I like your long “train of hollow lithium spheres, filled with liquid or frozen deuterium” concept.

I like the idea of using nuclear fusion in general for powering manned starships and no doubt, visionary folks like your self who can think outside of the box will help in the development of new kinematical scenarios involving applications of nuclear fusion for star flight.

What I did not consider, yesterday, was that the fuel can also be stored as Lithium deuteride. Lithium deuteride is used as primary fuel in the most powerful H-bombs (H being a misnomer here). See for example http://en.wikipedia.org/wiki/Castle_Bravo.

Lithium deuteride AFAIK is harder and thermally more stable than metallic lithium and thus probably better suited for most structural purposes. It is also a much better way to store the deuterium than its elemental form. The fact that it is highly reactive with oxygen and water (as is metallic lithium) is not much of a problem in space.

I do not know if lithium deuteride can be used effectively on a small scale for inertial confinement, and I do not think a magnetically confined reaction can be sustained with lithium and deuterium alone, as too many neutrons will escape. Thus, a lithium cooling/breeding blanket will most likely be required. In any case, it would not be too difficult to separate the lithium deuteride before use and feed the components into whatever fuel cycle is used.

Researchers at the University ofTexas at Austin plan to build an exawatt laser with a power equivalent to 1000 petawatts (1 quadrillion watts). The main research uses for their current laser, the Texas Petawatt Laser, the world’s most powerful, is to produce thermonuclear fusion for making electricity and to strike targets that release…

I do not lightly disagree with Freeman Dyson on his own subject but Orion is only “filthy” if low level radiation is significantly harmful. At the time even Dyson said it would only cause 1 extra cancer death worldwide, however even that depends on the LNT theory that there is no lower threshold to radiation damage. This is quite certainly wrong – there is considerable statistical evidecne that low level radiation ia actually beneficial (the hormesis theory) & NONE WHATSOEVER for LNT. Indeed there are many parts of the world such as Kerala in India & Yellowstone Park where radiation is & always has been well above “official” safe limits with no adverse effects detectable. In reality an Orion launch would be likely to save half a dozen lives worldwide by hormesis. This would not be its primary benefit.

“I lost all respect for anti-nuke groups during their tireade agains the launch of the Cassini probe to Saturn back in 1997.”

John Kerry and Barbara Boxer were the only two US Senators who were suckered by the anti-Cassini lies.

I don’t know what physicist Michio Kaku’s excuse was in that case, other
than perhaps a genuine if misguided concern over Cassini’s RTGs. At least
Carl Sagan showed that while he was deeply concerned about nuclear war,
he knew deep space missions needed nuclear power to work:

ljk said:
” I don’t know what physicist Michio Kaku’s excuse was in that case, other than perhaps a genuine if misguided concern over Cassini’s RTGs. ”

I think Kaku was trying to get some “face time” in front of the MSM in order to increase sales of his lousy books. If memory serves me correctly, Kaku actually claimed that if Cassini’s RTGs reentered the Earth’s atmosphere, the Pu-238 would scatter in the upper atmosphere and end all human life on the planet (Keep in mind that Kaku is a professorship in theoretical physics). Several American and Soviet spacecraft carrying RTGs with Pu-238 have reentered the Earth’s atmosphere. Apparently we have survived this repeated calamity.

ljk said: “I had not heard of Kaku before his tirade against Cassini and now he has more books out and even his own series on The Science Channel, so that much at least came true.”

I guess everyone has their price. Kaku’s price was significantly lower than typical for an established academic. Kaku has also appeared on the Art Bell show (talk about bottom feeding). Why anyone buys Kaku’s books or gives credence to his scientific theories is a mystery to me.

I think aneutronic energy should be pursued, because it can be much more practical and efficient than other fusion reactions. The aneutronic reactor can thrust a spaceship directly without radiation risk for the crew members.

Eniac said,
“But let’s not call something practical that has one person working on it and not a single peer-reviewed publication, much less a working proof of principle.”

R.W. Bussard’s group is still fiddling with their Polywell concept. Unfortunately R.W. Bussard passed away on 6 Oct 2007. The group doing the Polywell work got some Navy funding but I suspect their money will dry up before they produce a significant result. IMHO, aneutronic fusion is a no-hoper (ignition temperatures are too high).

Hi Gary,
I had a second cousin called Gary who visited Australia while working at the University of St Lucia in Queensland. (The mother of my cousin, Gary Allen, was Olga and his father’s name was William.)
My father, Paul Ussenko, came to Australia from Siberia via China. If you are that Gary Allen I would very much appreciate your emailing me as I am interested in doing some family research.
Many thanks,
Irene

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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